It’s because of the Tidal thing. Granted Rihanna has always had lower American album sales compared to her worldwide sales and its vice versus for Beyoncé
But the leak and the lack of promotion for Anti fukked it up. Then she didn’t release a video for one of the most popular songs on the album that was always performed on singing competitions.
Actually, both albums were Tidal exclusives. Rihanna was releasing teasers on the Fox network for weeks leading up to the album. She even had singles that ended up getting scrapped from the album.
And Beyonce hasn't always had lower global sales than Rihanna, but Rihanna has lagged behind Beyonce in the U.S.